I think I've read one's signature, but I forgot who own it. I think it's written like this: (please apologize if I got it wrong)
"Evolution doesn't deserve a creatures whether is the strongest one nor the brilliant one. It deserve the one who can adaptate quickly to gradual changes."

Q: Why are chemists great for solving problems?
A: They have all the solutions.

Since the loss of predatory maring reptiles such as the ichthyosaurs, sharks have really not had much competition in the realm of large predatory fish. Aquatic mammals such as killer whales and dolphins are really their only competition/rivals on a large scale anymore.
Like someone before me said, they found a good thing and stuck with it. Changes in environment or organisms around them hasn't necessitated any drastic changes yet.

What did the parasitic Candiru fish say when it finally found a host? - - "Urethra!!"

mithrilhack wrote:tuatara, crocodiles, etc...they're prolly as specialized as it gets...

All are also older species so they've had more time to adapt to their environment, and that is still working well for them so like everyone else said, no reason to change.
Other animals are evolving quicker because they need to, I'm sure sharks had to do the same thing.

Man in civilization surveys the creature through the glass of his knowledge and sees thereby a feather magnified and the whole image in distortion. - Henry Benson